The goal of this proposal is to identify new and better ways to treat leukemia. We propose to develop and employ a novel experimental system that will allow us to determine whether specific drugs or combinations of drugs will provide more effective forms of leukemia therapy. The biology of cancer stem cells has recently emerged as an important new consideration for the study of cancer and for developing new approaches to therapy. For the blood cancer leukemia, a role for malignant stem cells has been particularly well documented. Indeed, studies have demonstrated that a rare leukemic stem cell (LSC) can give rise to the bulk of myeloid (and some lymphoid tumors), and that such cells are biologically distinct from the majority of tumor tissue. Moreover, LSC appear to be refractory to conventional chemotherapy agents. Thus, elucidating the properties of LSC has become an important priority for leukemia research. To this end, some characteristics of human leukemia LSC have been described and approaches have been proposed to selectively eradicate LSC while sparing normal hematopoietic stem cells (HSC). However, to date, almost all direct knowledge of LSC is based on in vitro studies. No substantive analysis of the mechanisms regulating LSC survival in vivo has been reported. Thus, the primary objective of the proposal is to use a novel murine model to define methods for eradication of LSC in vivo. Studies in our lab have demonstrated that LSC can be readily identified in the model system and that their behavior closely mimics that of human LSC. In aim one of the proposal, we will employ the model to investigate the potential of various therapeutic agents to directly target LSC. In aim two, the model will be used to further define the cellular properties of LSC and to investigate mechanisms controlling drug resistance. Collectively, the proposed studies will provide a detailed evaluation of how therapeutic agents impact the growth and survival of LSC in vivo.